A touch-sensing-enabled active matrix LCD panel is an active matrix LCD display having an additional function of detecting a location of a possible touch contact made on the panel. The detection may be accomplished by an electrostatic capacitance method. This method is herein referred to as capacitive touch sensing method.
In the capacitive touch sensing method, large parasitic capacitance build-up is a common problem that affects the signal-to-noise ratio (SNR) of the touch-sensing signals generated from the touch sensors. The touch sensors are usually made of one or more layers of indium tin oxide (ITO) or metal. The parasitic capacitance build-up on the touch sensors is largely due to the close proximities between the touch sensors and various embedded electrodes in the LCD touch-display panel. These embedded electrodes include the display electrodes inside each display panel sub-pixel, source/data lines, scan/gate lines, and VCOM lines. Referring to FIG. 1. This parasitic capacitance includes CSource, CGate, and CVCOM, which can be in the order of 20˜100 pF.
One of the various capacitive touch sensing methods, self-capacitive sensing, is based on driving the sensor electrodes with an excitation waveform (hereinafter referred to as “sensor electrode excitation signal”), which is normally in the form of multiple voltage cycles, and in return detect the electric charge going forward and backward through the sensor electrodes as a means for capacitance measurement. The sensor electrode excitation signal provides the energy for toggling the touch sensors between a ground state and a predefined potential state. When a contact is made on the touch-display panel, extra capacitance (i.e. CFinger as shown in FIG. 1) is added to a touch sensor electrode, causing the electric charge going forward and backward through the touch sensor electrode to increase. Since these changes in capacitance caused by touch contacts are usually very small, in the order of 0.5 pF, the reduction of parasitic capacitance can substantially increase the analog dynamic range of the touch sensing. The better analog dynamic range in turn enhances the SNR of the digitized touch signals for better resolution using the same analog-to-digital converter (ADC) as in the case without the parasitic capacitance reduction.
Various methods have been developed with the goal of reducing parasitic capacitance. One such method is to drive the embedded electrodes with an excitation signal (hereinafter referred to as “embedded electrodes excitation signal”) that is similar to the sensor electrode excitation signal used for driving the touch sensors. By doing so, the voltage difference between the touch sensors and the embedded electrodes can remain constant, preventing the build-up of the parasitic capacitance. The present invention improves upon the current methods with varying voltage differences between the touch sensors and the embedded electrodes.